In mammalian skin, collagen is the most abundant structural constituent of the dermis, comprising of about three quarters of the dry weight of this part of the integument. The arrangement and tensile strength of the fibers and fiber bundles formed by this scleroprotein give the skin the ability to resist the mechanical stresses of tension and pressure. The limited elasticity, extensibility, tension and the shock absorption capacity of the skin depend on a three dimensional network of the collagen fibers and fiber bundles.
Collagen fibrils are made of many individual collagen molecules packed together, Each collagen molecule is a triple helix, three protein chains, each of which is a left-handed helix form and stable right-handed super-helix by hydrogen-bond, twisted together in a specific shape to form a sturdy, stable protein strand. When the water content of animal skin is relatively low, collagen protein is very stable and cannot be decomposed by heating at normal temperatures in a short time. However in high water content, the collagen protein can be denatured by heating. In this state, hydrolytic cleavage of covalent bond loosen the hydrogen bond or static bond in the collagen protein, and the three tropocollagen strands will be separated partially or completely into globular domains, containing a different, secondary, structure to the normal collagen polyproline II (PPII), e.g. random coils, i.e. collagen is denatured into gelatin. It is difficult to reorganize the modified animal skin into new skin with similar features such as glutinosity, flexibility and tensile properties as natural skin.
For years people have produced artificial collagen membranes and casings with modified natural animal collagen. So far, the major source of the animal collagen has been from bovine collagen from bovine hide. After butchering the cow and peeling off the skin, the underlayer of bovine collagen will be separated and mechanically transformed into a gel. The reformed gel will be further extruded to form casings, and subsequently the casings will be solidified, typically by adjusting the pH and/or applying a cross linker e.g. glutaraldehyde. However, pig collagen is seldom used to produce artificial collagen casings, or artificial skin, mainly because of the low tensile strength of the product.
The general method for producing artificial animal skin using pig skin usually includes the next steps: (1) washing the skin with alkali; (2) grinding the skin; (3) adding additives such as polyvinyl alcohol aldehyde condensation compound, etc., and (4) extruding the mixture into shapes and obtaining tubular casings by a method of pushing and bath-law concreting. Through this method, a 0.03-0.1 mm sausage casing is finally produced in a sequenced process of drying, modifying, folding or shrinking.
T. Morgan also disclosed a method (Chinese Patent Application No. 02816630.2) of extruding the gel containing pig collagen, fat and moisturizing agent to produce tubular casings for foods (e.g. sausage). However, this method is relatively complicated: firstly, it comprises a step of adjusting the proportions of collagen, fat and moisturizing agent in the materials, and secondly, it is a complicated de-fatting process.
In view of the above mentioned defects in prior art, this invention is dedicated to provide a new method of producing products with sufficient tensile strength using animal skin, especially pig skin. The products can be used as pet foods, e.g. dog foods, or pet toys, or packaging materials.